1 For the sponge base measure out all the ingredients in advance.
2 Grease the base of the springform pan (26 cm).
3 Beat the egg yolks with half the sugar and the vanilla sugar until thick and creamy.
4 Whisk the egg whites, trickling in the rest of the sugar before it becomes too stiff, then continue whisking until completely stiff.
5 Fold the beaten egg whites into the egg yolk mixture with a spoon
6 Sieve the flour and cornflour on to the egg mixture and fold in.
7 Turn the mixture into the prepared springform pan, smooth the top and bake in a preheated oven (175°, middle shelf) for 25-30 minutes. (Test for readiness by inserting a needle)
8 When it is cooked, take the sponge base out of the oven, cool slightly and carefully loosen from the edge of the springform pan using a thin knife. Turn out on to a cake rack to cool and leave to rest for at least 2 hours. Then split twice (making three layers).
9 Mix the quark with the sugar, vanilla sugar, 2 tbsp orange juice and about 1 tbsp lemon juice. Whip the cream stiffly and fold in.
10 Peel the avocado, remove the stone and slice thinly. Sprinkle with lemon juice at once.
11 Cut the watermelon flesh into thin triangular slices.
12 Divide each layer of sponge into 2 strips of about 6 x 16 cm. (use the leftover sponge for another purpose). Cut each strip in half and then diagonally into triangles. Sprinkle all but 8 sponge triangles on one side with a little orange juice. Spread a little quark cream on the unsprinkled triangles, top with avocado and cover with sprinkled triangles. Spread with more quark cream and top with watermelon.
13 Place the remaining triangles, sprinkled side down, on top of the watermelon. Press down lightly.
14 Garnish the sandwiches with lemon balm and dust with icing sugar.